1. Field of the Invention
This invention relates generally to rotary vane pumps or motors and in particular to vane pumps having variable displacement.
2. Description of the Prior Art
There are many different types of rotary vane pumps, most of which employ an externally rotated shaft that is offset within the axis of the pump housing and has a series of equally spaced radial slots each having a spring biased paddle or pump vane. The outer end of the vanes brush against the circular interior surface of the housing to force fluid from an intake port to an output port. If used as a motor, such a pump will receive steam, compressed gas or pressurized liquid to force rotation of the vane assembly.
A second type of rotary vane pump operates in the same manner but employs a rotary vane assembly having a rotating axis that is offset from the axis of the powered input shaft. The base of each vane is freely hinged to the vane assembly axis centered in the circular housing and rotatable in a bushing in one side of the pump housing. The hinged vane assembly is rotated by a rotor coupled to the input shaft entering the pump housing from the opposite side. Since the vane assembly axis is displaced from the input shaft axis, the vane ends force fluid from inlet to outlet in the same manner discussed above. A typical rotary van pump or motor of this type is described in U.S. Pat. No. 3,892,502 of Pritchard, dated July 1, 1975.
An improvement to the rotatable vane pump with the offset shaft and vane assembly axes described above is one in which the amount of offset may be varied, such as described in U.S. Pat. No. 3,807,912 of Keller, dated Apr. 30, 1974. Here, a radially aligned rack operated by an external pinion gear adjusts the diametrical position of a secondary or interior circular housing with a hinged rotatable vane assembly centered therein. The pump input shaft rotates a rotor assembly which slideably engages each vane for rotating the vane assembly. The advantage of such a pump over those discussed above is that its displacement is variable so that at a constant rotational input velocity it may control flow and pressure and also the direction of flow.
The input shaft of Keller's device moves relative to the primary pump housing. This movement requires a second, larger housing for containment purposes. The primary pump housing must be fairly massive to withstand several thousand P.S.I. of internal pressure. When a second housing is added around the primary housing, Keller's device becomes too large for use in many applications.
Furthermore, Keller achieves variable displacement by moving a "control device" subassembly back and forth by means of a rack and pinion. The control device divides the outer cavity into low pressure and high pressure regions, which results in large lateral forces on the control device. Due to the large, unbalanced forces exerted on the control device, it must be made massive and rigid, which adds to the cost and weight of Keller's device.